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The .55 Boys is basically a .50 BMG cartridge necked up to accept a .55 caliber bullet and has a belt added to its case. Average muzzle velocity is 2,495 fps for the AP MK I, one of the most common variants of this cartridge. It was designed to be an anti-tank cartridge, which, as a concept, was started with the World War I-era 13.2mm TuF round. However, its performance is quite low compared to other anti-tank rounds of the time, such as the 7.92 x 94 Patronen and the 14.5x114mm rounds.[1] This eventually lead to it being obsolete almost immediately after the United Kingdom entered World War II.

In the 1930s, the United Kingdom decided that they needed an anti-tank rifle to counter enemy tanks in a future war. Early work on a 13.2mm round was started as a base, most likely because the first mass-produced anti-tank cartridge, the 13.2mm TuF, used a 13mm caliber bullet. However, the idea of a 13.2mm round was eventually abandoned.[2]

Development on what is known as the .55 Boys was started by Captain H C Boys, a designer at the Royal Small Arms Factory, Enfield. The basic idea behind the .55 Boys seems to be that it would be a modified .50 BMG round necked up to accept a larger, steel core bullet in order to increase its armor penetration. A belt was also added to ensure the round could not be chambered in arms designed for the .50 BMG.

The .55 Boys was adopted with the Boys Anti-Tank Rifle in 1937, and was manufactured with the Boys Anti-Tank Rifle throughout the Commonwealth of Nations by firms such as Kynoch. However, when the United Kingdom entered World War II, the .55 Boys round was soon found to be lacking against even the tanks used by enemy forces in 1939.[1] However, the United Kingdom was forced to use the .55 Boys round for several years because no better infantry anti-tank weapons were available. When the PIAT was introduced, the shaped charges it fired proved to be far more effective against enemy armor than the .55 Boys round ever could.[1] This essentially sealed the .55 Boys fate to become obsolete. Despite its relative ineffectiveness, the .55 Boys was used throughout World War II in both the Pacific and Atlantic theaters and also saw use during the Winter War and Continuation War by Finland. However, once World War II was finished, the .55 Boys was no longer used in any major capacity and thus quickly faded to obsolescence.

This is the first variant of the .55 Boys. It uses a 926 gr. hardened steel core bullet with a lead sleeve, which is covered with a steel jacket. A ball and tracer versions of this round were also created. A practice round using an aluminum core were also produced in order to make it more feasible for training.

An experimental APCR (Armor Piercing Composite Rigid) .55 Boys round was designed in 1942.[5] It used a tungsten core round instead of a steel core, which greatly increased its penetrating ability and gave a boost to its muzzle velocity from the Mark II's 2,900 feet per second to 3,100 feet per second. It can easily be discerned from normal Mark I and II rounds because of its two-part bullet. The Mark II was never officially adopted because far better anti-tank rounds and weapons, such as the PIAT, were entering service at the time. The .55 Boys, even with a greatly improved bullet, was simply too weak to defeat the tanks being fielded at the time.